Gas turbine engines operate by burning fuel and extracting energy from the combusted fuel to generate power. Atmospheric air is drawn into the engine from the environment, where it is compressed in multiple stages to significantly higher pressure and higher temperature. A portion of the compressed air is then mixed with fuel and ignited in the combustor to produce high energy combustion gases. The high energy combustion gases then flow through the turbine section of the engine, which includes a plurality of turbine stages, each stage comprising turbine vanes and turbine blades mounted on a rotor. The high energy combustion gases create a harsh environment, causing oxidation, erosion and corrosion of downstream hardware. The turbine blades extract energy from the high energy combustion gases and turn the turbine shaft on which the rotor is mounted. The shaft may produce mechanical power or may directly generate electricity. A portion of the compressed air is also used to cool components of the turbine engine downstream of the compressor, such as combustor components, turbine components and exhaust components.
In some gas turbine engines, the compressor discharge casing is a complex cast iron structure that locates the combustion hardware (e.g. fuel nozzle, combustion liner and transition pieces) between the compressor exit and the turbine inlet. Air from the compressor is a permitted to leak around the compressor discharge casing to cool the region in front of the first rotor and turbine blade set mounted on the rotor, also referred to as the first forward wheelspace (1FWSP). Of course, the amount of cooling air is determined based on the pressure of the compressor discharge air, which can vary at fixed load conditions based on ambient air temperature. To provide additional cooling, boreplugs are provided in the compressor discharge casing that permits additional compressor discharge air to flow into 1FWSP to provide additional cooling. The number of boreplugs to be opened is based on anticipated cooling flow requirements. If the anticipated cooling flow is incorrect, then cooling either will be inadequate, causing the temperatures in the 1FWSP to be too high, which can result in shortened life expectancy of the components being cooled, or will be excessive, resulting in the unnecessary diversion of compressor air that can result in operational inefficiency. Of course, because the boreplugs are opened or removed on installation based on anticipated cooling flow, correction of the cooling flow by addition or removal of plugs must await maintenance, as removal of a gas turbine from service to accomplish this modification is not cost effective.
Due to rising fuel costs, natural gas fired power plants that were designed to operate at mostly full power output are now being operated on a intermittent basis. Coal and nuclear energy now generally make up the majority of stable power output. Gas turbines are being increasingly used to make up the difference during peak demand periods. For example, a gas turbine may be used only during the daytime and then taken off line during the night time when the power demand is lower. During load reductions or “turndowns”, gas turbines typically can remain in emissions compliance down to about forty to forty-five percent (40% to 45%) of full rated load output. Below this load, carbon monoxide (CO) emissions can increase exponentially and cause the system as a whole to go out of emissions compliance. Generally described, emissions compliance requires that the turbine as a whole to produce less than the guaranteed or predetermined minimum emissions levels. Such levels may vary with the ambient temperature, system size, and other variables. Especially the turndown capability of the gas turbine goes down in cold ambient, i.e. as the ambient temperature falls, the minimum load for CO compliance rises steeply. If a gas turbine has to be shutdown because it cannot remain in emissions compliance due to a low power demand, the other equipment in a combined cycle application also may need to be taken offline. This equipment may include a heat recovery steam generator, a steam turbine, and other devices. Bringing these other systems online again after a gas turbine shutdown may be expensive and time consuming. Such startup requirements may prevent a power plant from being available to produce power when the demand is high. There may be a strategic operational advantage in being able to keep a gas turbine online and in emissions compliance during periods of low power demand so as to avoid the start up time and expense. The above defined minimum load is a function of combustion temperature. If the combustion temperature drops down below a predetermined value, the CO emission increases. This temperature is a function of fuel air ratio in the combustor. So during gas turbine load reduction the fuel and air flow has to be reduced proportionately to maintain required combustion temperature. Current gas turbine design have several limitation on minimum allowable air flow to the combustor below a predetermined gas turbine load which impacts the fuel air ratio also the combustion temperature and increases the emission at lower gas turbine load. There is a desire therefore for methods to minimize the air flow to the combustor further as function of fuel flow at lower loads and extending gas turbine emissions compliance during periods of reduced loads.
Shape memory alloys (SMA), sometimes referred to as smart materials, have the ability to change shape based on microstructure and composition. SMAs take advantage of the transition of the microstructure from a low temperature martensitic structure to a high temperature austenitic structure (and back) in a predictable manner. The SMAs may provide the ability to regulate the airflow through boreplugs by opening, closing (or partially opening) the bore apertures thereby increasing or decreasing airflow. And while one well-known SMA, nitinol, or NiTi having roughly an equal atomic percentage of Ni and Ti, is unsuitable for use as a boreplug opening due to the high temperatures experienced in the operation in a gas turbine engine, other SMAs having the ability to survive high temperatures of operation as well as the corrosive, oxidative environment of a gas turbine engine may be suitable. Thus, a shape memory alloy suitable for use in the high temperature, oxidative and corrosive environment of a gas turbine engine may find use as a component for the regulation of cooling flow based on changing operational conditions.